Transformation of the qualitative composition of soil organic matter in podzolic chernozem under the application of composts

Abstract

Given the important role of soil organic matter (SOM) in the global carbon cycle and its potential feedbacks under fertilizer application, understanding how organic matter composition and molecular structure of humic substances responds to prolonged organic fertilization is of great scientific interest. The aim of this study was to investigate the transformation of humic composition in chernozem organic matter using Tyurin fractionation, alongside structural characterization of isolated humic acids by size-exclusion chromatograph and polyacrylamide gel electrophoresis (SEC-PAGE) techniques, following a five-year application of organic fertilizers based on composted chicken manure. We found that the intensive formation of “young,” labile humic substances as first stage of humification occurred after the first year of manure compost application, accompanied by an increase in soil organic carbon. After five years of organic manure application, the organic matter fractions became more stabilized, as evidenced by an increase in mature humic fractions but accompanied by a decrease in the proportion of labile fractions. The increasing molecular complexity of chernozem humic acids after five years of manure compost application may be attributed to a higher content of stable low-molecular-weight molecules in compost, such as aromatic non-polar structures that strong absorb in ultraviolet. The most complex humic molecular structures were found from chernozem subjected to application of composted manure and husk, which contained a high abundance of condensed aromatic already formed humic structures. This molecular complexity may result from active hydrophobic humification, accompanied by the self-assembly of amphiphilic condensed aromatic structures into stable and complex hydrophobic humic supramolecular aggregates. These aggregates bind with calcium and subsequently with mineral clays, leading to the formation of mature humic fractions promote organic matter stabilization effect. We demonstrate the applicability of size-exclusion chromatograph and polyacrylamide gel electrophoresis fractionation techniques for evaluating the molecular complexity of humic structural organization in agricultural soil under organic fertilization by identifying low-molecular-weight humic fractions.